Processing of metal chloride solutions and method and apparatus for producing direct reduced iron

ABSTRACT

A process and apparatus for regeneration of acid and metal from spent acid liquors includes the steps of, optionally, concentrating ( 110 ) the liquor to a concentrated liquor or solid, pyrohydrolysing ( 114 ) to regenerate the acid and form metal oxide pellets, and reducing the oxide to metal in a two stage reduction reactor ( 116, 118 ) using a partially combusted fuel as reducing agent. Also disclosed is a two-stage reduction process and reactor for production of direct reduced iron (DRI) from iron oxide ores or wastes, including a first stage ( 416 ) in which the oxide feed is contacted with a fuel and a sub-stoichiometric amount of an oxygen source to produce a lower oxidation state oxide, and a second stage ( 418 ) in which the lower oxidation stage oxide is contacted with off-gases from the first stage to produce iron metal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for processingmetal chloride liquors, such as spent acid liquors from either acidleaching of metalliferous minerals or acid pickling of metals, and to amethod and apparatus for production of direct reduced iron (DRI).

2. Description of Related Art

There are several important industrial processes which produce as aby-product metal chloride liquors, in the form of spent acid liquorswhich contain metal chloride in solution.

Such metal chloride liquors include:

-   -   Spent leach liquors resulting from the acidic digestion of a        mineral species such as, but not necessarily restricted to, the        mineral known as ilmenite. The acid in this case may be, but is        not necessarily restricted to, hydrochloric acid.    -   So-called ‘pickle liquor’ resulting from an acid treatment or        pickling, or etching of metallic surfaces for cleaning of rust        or oxide scale. The acid in this case may be, but is not        necessarily restricted to, hydrochloric acid.

In the first of these cases, ilmenite, a titanium mineral, is leached inhydrochloric acid to yield a synthetic rutile and spent leach liquorcontaining iron and other chlorides in solution, together with a smallexcess of hydrochloric acid, according to the generalised reactionequation given below:

FeO.TiO₂+2HCl→FeCl₂+TiO₂+H₂O

Elemental oxide constituents of the ilmenite (other than iron) which aresoluble in hydrochloric acid are similarly dissolved to form theirrespective chlorides. The resultant solution from this type of reactionwould typically have a composition similar to the example given in thetable below:

TABLE 1 Typical analysis of spent metal chloride liquor from acidleaching of ilmenite. Compound Mass % grams/litre TiCl₄ 0.08 1.14 FeCl₂27.37 368.59 CrCl₃ 0.03 0.44 MgCl₂ 0.56 7.55 CaCl₂ 0.26 3.56 MnCl₂ 0.293.90 SiCl₄ 0.02 0.21 VCl₅ 0.11 1.52 AlCl₃ 0.56 7.57 HCl 3.71 50.00 H₂O67.00 902.46

In the second case, steel products that have acquired an oxide or rustcoating are often subjected to a process referred to as ‘pickling’wherein the oxide or rust coating is removed by passing the steelthrough a hot solution of hydrochloric or other acid. In the case of theuse of hydrochloric acid, the reactions involved may be simplified tothe following:

Fe₂O₃+Fe+6HCl→3FeCl₂+3H₂O

Known processes for treatment of such metal chloride liquors concentrateon regeneration and recovery of the acid, for recycling to the acidleaching or pickling process.

This acid regeneration is commonly done by pyrohydrolysis, in which themetal chloride reacts with water and oxygen to recover the acid,producing a metal oxide as a by-product.

In the case of iron chloride, the predominant reactions taking placeduring pyrohydrolysis may be expressed as follows:

2FeCl₂+2H₂O+ 1/2O₂→Fe₂O₃+4HCl

Pyrohydrolysis is conducted at temperatures which may range from 600° C.to 1200° C. but preferably in the range of 850° C. to 950° C. Thefluidising gas is typically air. Sufficient fuel is added to maintainreaction temperature and to control oxygen potential. The off-gas,containing products of combustion and hydrochloric acid vapour, istreated for recovery of the hydrochloric acid component.

Pyrohydrolysis may be carried out by one of two well-known methodsutilised in the current art, namely:

-   -   In a spray tower, wherein the liquor is sprayed into a void        chamber and reacted with hot gases derived from the burning of a        liquid or gaseous fuel. In this instance the metal oxide product        is in the form of a fine powder.    -   In a fluidised bed reactor, wherein the liquor is injected into        a fluidised bed of previously manufactured metal oxide, and the        mass maintained at high temperature by the injection of a        gaseous, liquid or solid fuel or any mixture thereof. In this        type of equipment, the metal oxide residue may be in the form of        small pellets with a nominal size of 1 mm to 2 mm in diameter.

As a further development, as instanced in International PatentApplication PCT/AU93/00056 (WO93/16000), the metal chloride liquor isfirst evaporated to a dry pelleted form before feeding to apyrohydrolysis reactor of the fluid bed type. The evaporator in thisinstance may preferably be of the fluid bed type or alternatively rotaryequipment may be used, according to normal practice for the thermalevaporation of water from salts in solution.

The process of WO93/16000 is particularly advantageous for acidregeneration in acid leaching processes as it results in a highlyconcentrated, superazeotropic, acid solution for return to the leachingstep.

The contents of WO93/16000 are incorporated herein by reference.

Other known acid regeneration processes include the KeramChemie process,in which the metal chloride liquor undergoes pyrohydrolysis withoutpre-concentration, resulting in a sub-azeotropic acid solution.

SUMMARY OF THE INVENTION

The present invention aims to provide a process of improved commercialviability, and apparatus for conducting the process.

In one form, the invention provides a process for regeneration of acidand metal from spent acid liquor containing metal chloride in aqueoussolution, including the steps of, in sequence:

-   -   Optionally, concentrating the liquor to form a liquor        concentrate or solidifying the liquor concentrate to form a        solid metal chloride;    -   Pyrohydrolysing the liquor, concentrated liquor or solid metal        chloride in a reactor to regenerate the acid and form a metal        oxide; and    -   Reducing the metal oxide to metal.

Preferably, the metal is iron or is predominantly iron.

In one preferred form, the reduction step is conducted as a two stagereduction reaction, including a first stage reduction reaction to alower oxidation state oxide, using a partially combusted fuel as areducing agent, and a second stage reduction reaction in which the loweroxidation state oxide is converted to the metal.

Preferably, the first reduction stage is conducted in a first fluid bedreduction reactor in which the metal oxide is contacted with the fueland a sub-stoichiometric amount of oxygen.

Alternatively, the fuel and oxygen may be contacted prior to the firstreduction stage.

Preferably also, the off-gas of the first reduction stage is used as thereducing agent for the second reduction for the second stage.

In a further preferred form, the off-gas from the second stage reductionreactor is oxidized to provide energy for the concentration orpyrohydrolysis steps of the process. Preferably the hot gas required forthe evaporation of the metal chloride liquor is provided by theoff-gases from the oxide pellet metallising stage.

In a further form, the invention provides a process for treatment of ametal oxide feed, including the steps of:

-   -   Contacting the metal oxide feed with a reducing agent comprising        a fuel and a sub-stoichiometric amount of oxygen, in a first        stage reduction to form a lower oxidation state oxide; and    -   Feeding the lower oxidation state oxide formed by the first        stage reduction and off-gases from the first stage to a second        reduction stage wherein the lower oxidation state oxide is        reduced to the metal.

Preferably, the first and second stages reduction reactions are carriedout in respective first and second fluid bed reduction reactionchambers.

Preferably, the metal oxide feed is formed by pyrohydrolysis of a spentacid liquor containing a metal halide, preferably chloride.

Further aspects of the invention are as set out in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further preferred embodiments of the invention will now be describedwith reference to the accompanying drawings, in which:

FIG. 1 is a flowchart of processing of a spent metal chloride liquor inaccordance with a first embodiment of the invention;

FIG. 2 is a flowchart of processing of a spent metal chloride liquor inaccordance with a second embodiment of the invention;

FIG. 3 is a flowchart of processing of a spent metal chloride liquor inaccordance with a third embodiment of the invention, and

FIG. 4 is a flowchart of processing of an iron ore to iron metal.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a first embodiment of the invention, which includesevaporation and pelletisation of the metal chloride (MeCl) liquor priorto pyrohydrolysis.

Spent leach liquor derived from the leaching of iron or other metallicoxide or spent pickle liquor derived from a steel or metal finishingprocess is injected into an evaporator 110 of a fluid bed, rotary kilnor other suitable type. The iron or metal chloride product dischargedfrom the evaporator is preferentially a pelleted solid. The hot gasproviding the heat source for this evaporation is derived from theoff-gas of the two metallising stages hereinafter described, furtheroxidised in an afterburner section 112 of the evaporator.

The evaporator product is fed to a pyrohydrolysis reactor 114, normallyof the fluid bed type, where the iron or other metal chlorides areconverted to metallic oxides by reaction with water at a temperaturepreferably in the range of 600° C. to 1200° C. The water for thisreaction may be provided by combustion of the fuel used and by residualwater of crystallisation in the reactor feed from the evaporator.

The concentration, pelletisation and pyrohydrolysis steps and theirequipment and operating parameters are known per se, and are describedin more detail in WO93/16000.

It should be noted that spent liquor or concentrated spent liquor may besubstituted for part or all of the pelletised material fed to thepyrohydrolysis reactor.

Metal oxide pellets are discharged at a high temperature from thepyrohydrolysis reactor into the a fluid bed gasifying reactor (gasifier)116, which serves as the first stage reduction reactor, operating atapproximately 1000° C.±100° C. The gasifier may be supplied with eithera solid, liquid or gaseous hydrocarbon fuel such as is appropriate tolocal availability and cost structures. Suitable fuels may include coal,oil, or natural gas.

In the gasifier, the fuel is converted to a reducing gas predominantlyconsisting of hydrogen and carbon monoxide.

As a further embodiment, the feed to the first stage reduction unit maybe augmented or (as discussed later with reference to FIG. 4) entirelyreplaced by iron ore or metallic oxide wastes such as mill scales andbaghouse dusts. Alternatively, or in addition, metallic oxide wastes maymixed with the metal chloride liquor before the evaporation step.

The gasifying reactions involved are quite complex but have beenexhaustively studied and reported in the literature. Examples of some ofthe more simple reactions involved are given below:

C+O₂→CO₂

2C+O₂→2CO

2C+1½O₂→CO+CO₂

2C+3H₂O→CO+CO₂₊₃H₂

CH₄+2O₂→CO₂+2H₂O

CH₄+H₂O→CO+3H₂

The oxygen for these reactions may be supplied either as ambient orpreheated air and with or without oxygen enrichment. The aim is toensure that only sufficient oxygen is used to provide enough heat ofcombustion to maintain the endothermic reactions that generate the COand H₂ components necessary for the metal oxide reduction reactionsoccurring simultaneously. For example, it has been found thatsatisfactory results may be obtained with an oxygen supply of between30% to 50% of the full stoichiometric combustion requirement.

The bed of FeO acts as a catalyst for the gasification reactions.

In the gasifier, the metal oxide is converted rapidly, in the case ofiron (III) oxide, into an oxide of lower valence, Fe(II), withoutmetallising. It is important that the reduction takes place rapidly sothat the formation of FeO is predominant and the formation of theintermediate oxide Fe₃O₄ is minimised, as too high a proportion of Fe₃O₄may lead to incipient fusion taking place and the resultant ‘stickiness’leading to de-fluidisation of the bed.

The reactions taking place in the first stage reduction are:

3Fe₂O₃+H₂→2Fe₃O₄+H₂O

3Fe₂O₃+CO→2Fe₃O₄+CO₂

Fe₃O₄+H₂→3FeO+H₂O

Fe₃O₄+CO→3FeO+CO₂

From the gasifier, partially reduced solid oxide pellets—predominantlyFeO—are elevated by means of a pneumatic “J-valve” fluid power or othersuitable device into the metallising fluid bed reactor train 118. Thegases exiting the first stage reactor are used as the fluidising mediumin the second stage or metallising reactor and contain sufficientresidual reducing gases H₂ and CO for the required reaction. Where coalis used as the fuel for the first reduction stage, char may be formed inthe first stage and is carried with the oxide pellets into the secondstage. The temperature of the second reactor chamber is slightly lowerthan in the first reactor—for example about 900° C.±100° C.—and theresidence time in this reactor is of the order of one hour for maximumconversion of oxide to metal.

The reactions in the second stage reduction are:

FeO+CO→Fe+CO₂

FeO+H₂→Fe+H₂O

By using the off-gases from the first stage for the second stagereduction, the minor proportion of Fe₃O₄ in the reaction products fromthe first stage is reduced by reaction with the CO.

Fe₃O₄+CO→3FeO+CO₂

FeO+CO→Fe+CO₂

The product from the metallising stage is indirectly cooled under suchconditions as to exclude air and so avoid any reoxidation of the productwhich could occur whilst the material is at an elevated temperature.Nominally, the metallised pellets must be cooled to less than 200° C. orlower before contact with air is allowed.

The indirect solids cooler 20 may be of any suitable type, as known inthe art. Where air is used as an indirect cooling medium, as it is inthe illustrated cooler, the resultant hot air may be used as the airfeed to the gasifier 116.

As mentioned above, the hot gas required for the evaporation of themetal chloride liquor is provided by the off-gases from the oxide pelletmetallising stage. The gases are first passed through the afterburner112 where any excess carbon monoxide and hydrogen are converted to extraheat for use in the evaporator. Furthermore, heat may be recovered fromthe hot metal pellets for use in preheating the air feed to the firststage reduction reactor or elsewhere in the plant.

As a result, it is expected that the metallisation step adds less than20%, and most likely only about 10%, to the total fuel requirements ofthe process, compared to the process of WO93/16000, but with asubstantial increase in the economic value of the end products.

Furthermore, the above process provides a single solution to the steelindustry for processing of pickle liquor and recycling of iron oxidewastes such as mill scale and baghouse dust, producing regenerated acidand direct reduced iron as valuable products. Higher zinc baghouse dustsmay be added to the spent liquor stream for processing and bound in theinert iron oxide pellets produced by the pyrohydrolysis step fordisposal without metallisation.

The acid regeneration plant also has the capability to handle wastewater streams generated during normal steelmaking operations or fromsite storm water run-off. This water may contain fine oxides, oil (fromrolling mills), fine coal/carbon or chlorides and would be fed into theevaporator or used for acid absorption, depending on the contaminantspresent.

The process produces no solid or liquid effluents, and dioxins andfurans have been below the level of detection during test operations.

Operating Data

During a bench-scale trial with spent liquor derived from thehydrochloric acid leaching of an ilmenite, the following data wasrecorded:

Liquor Feed to the Evaporator

The composition of the feed liquor to the evaporator has been quoted inTable 1 above

Evaporation Stage

Evaporation and drying of the solid iron chloride was taken to the pointwhere the so-called water of crystallisation corresponded to anempirical formula of FeCl₂.1.5H₂O, thus providing sufficient water forthe pyrohydrolysis reaction

Pyrohydrolysis

Pyrohydrolysis was conducted at a nominal temperature of between 850° C.to 950° C.

Reduction

Iron oxide pellets from pyrohydrolysis were first reduced to themonovalent state and then fully reduced to the metallised state. Therelevant data are given in Table 2 below.

The temperature in the first stage was 950° C. and the retention timewas half an hour.

The temperature in the second stage was 930° C. and the retention timewas two and a half hours.

TABLE 2 Reduction Feed and Product analyses Oxide from MetallisedCompound Unit Pyrohydrolysis Product TiO₂ % mass 0.11 0.16 Fe₂O₃ ″ Nil{close oversize brace} 95.5 FeO ″ 6.0 Fe ″ Nil 89.7 Cr₂O₃ ″ 0.06 0.08MgO ″ 0.84 1.18 CaO ″ 0.02 0.03 MnO ″ 1.64 2.29 SiO₂ ″ 0.13 0.18 V₂O₅ ″0.60 0.84 Al₂O₃ ″ 0.38 0.54 P₂O₅ ″ 0.004 0.005 Nb₂O₅ ″ 0.002 0.003 Slagindex ratio Not applicable 1.7 Sponge Iron % mass Not applicable 1.6

The iron product from this trial was non-pyrophoric; therefore nobriquetting or special handling would be required for subsequent safeuse.

The chemical composition and physical state were such that it would beconsidered satisfactory feed for molten metal or steel production, forexample as feedstock for an electric arc furnace.

Operating Pressures

This embodiment of the invention operates at essentially atmospheric orlow pressure and none of the major process vessels or equipment need becertified according Australian Standard AS 1210-1997 and any amendmentsthereof.

FIG. 2 illustrates a second embodiment of the invention, in which thespent metal chloride liquor is fed directly to the pyrohydrolysisreactor 214 without prior evaporation and pelletisation.

In the embodiment of FIG. 2, the illustrated process are constructed andoperated in similar manner to their analogously numbered componentsdescribed above with reference to FIG. 1, except that the pyrohydrolysisreactor typically will need to be larger due to the more dilute metalchloride feed liquor.

In the arrangement of FIG. 2, the off-gases from the stage 2 reductionare fed to an afterburner 212 and waste heat recovery unit 213 and thewaste heat is recovered for use elsewhere in the plant.

In the arrangement of FIG. 3, the off-gases from the stage 2 reduction318 are fed directly to the pyrohydrolysis unit 314 for furthercombustion and recovery of heat values.

In FIGS. 2, 3 and 4 the reference numerals are allocated by analogy tothose of FIG. 1, with ‘200-series’ numerals used in FIG. 2, ‘300-series’numerals used in FIG. 3 and ‘400-series’ numerals used in FIG. 4.

FIG. 4 illustrates an alternative embodiment, in which iron ore fines oriron oxide wastes are reduced to iron.

The iron ore or oxide fines—for example of diameter about 3 mm—are fedto a fluid bed preheater 422, using the residual chemical and thermalenergy in the off-gases from the second stage reduction reactor 418. Theoff-gases, which contain CO and H₂, are fed to an afterburner 424 in thebase of the preheater 422 and the combustion gases fed to the fluid bed.

The preheated feed is then fed to the two-stage co-current reductionreactor 416, 418, which operates as described above with reference toFIGS. 1 to 3.

Though the present invention has been described above with respect to aparticular embodiment thereof it is to be understood that the inventionis not limited thereto but is capable of variation within the knowledgeof a person skilled in the art. All changes which come within themeaning and range of equivalency of the claims are intended to beembraced therein.

In this specification, the word “comprising” is to be understood in its“open” sense, that is, in the sense of “including”, and thus not limitedto its “closed” sense, that is the sense of “consisting only of”. Acorresponding meaning is to be attributed to the corresponding words“comprise”, “comprised” and “comprises” where they appear.

It will further be understood that any reference herein to known priorart is not, unless the indicated to the contrary, an admission that suchprior art is commonly known by those skilled in the art to which theinvention relates.

1. A process of regeneration of acid and metal from spent acid liquorcontaining metal chloride in aqueous solution, including the steps of,in sequence: (a) optionally, concentrating the liquor to form a liquorconcentrate or solidifying the liquor concentrate to form a solid metalchloride; (b) pyrohydrolysing the liquor, concentrated liquor or solidmetal chloride in a reactor to regenerate the acid and form a metaloxide; and (c) reducing the metal oxide to metal.
 2. A process accordingto claim 1, wherein the metal is iron or is predominantly iron.
 3. Aprocess according to claim 1, wherein the reduction step is conducted asa two stage co-current reduction reaction.
 4. A process according toclaim 3, wherein a first stage reduction reaction to a lower oxidationstate oxide uses a partially combusted hydrocarbon fuel as a reducingagent.
 5. A process according to claim 4, wherein the first reductionstage is conducted in a first fluid bed reduction reactor in which themetal oxide is contacted with the fuel and a sub-stoichiometric amountof oxygen.
 6. A process according to claim 4, wherein said oxygen isprovided to said first reduction stage as air.
 7. A process according toclaim 4, wherein off-gas of the first reduction stage is used as thereducing agent for the second reduction for the second stage.
 8. Aprocess according to claim 3, wherein off-gas from the second stagereduction reactor is oxidized to provide energy for the concentration orpyrohydrolysis steps of the process.
 9. A process according to claim 1,wherein off-gases from the oxide pellet metallising stage are used forthe evaporation of the metal chloride liquor.
 10. A process according toclaim 2, further including the step of adding iron oxide to the aqueoussolution of metal chloride.
 11. A process for treatment of a metaloxide, including the steps of: (a) contacting the metal oxide feed witha reducing agent comprising a fuel and a sub-stoichiometric amount of anoxygen source, in a first stage reduction to form a lower oxidationstate oxide; and (b) feeding the lower oxidation state oxide formed bythe first stage reduction and off-gases from the first stage to a secondreduction stage wherein the lower oxidation state oxide is reduced tothe metal.
 12. A process according to claim 11, wherein said metal isiron.
 13. A process according to claim 12, wherein lower oxidation stateoxide is predominantly FeO.
 14. A process according to claim 10, whereinthe first and second stages reduction reactions are carried out inrespective first and second fluid bed reduction reaction chambers.
 15. Aprocess according to claim 14, including transporting said loweroxidation state oxide from the first reaction chamber to the secondreaction chamber by means of an inert gas stream.
 16. A processaccording to claim 12, wherein said metal oxide feed is an iron ore. 17.A process according to claim 11, wherein said metal oxide feed is apelletised metal oxide formed by pyrohydrolytic regeneration of a spentacid liquor containing a metal halide
 18. A process according to claim12, wherein said metal halide is iron chloride.